1. Field of the Invention
This invention relates to a structure floating on a body of water. More particularly, the invention relates to a floating structure from which drilling wells and/or production of oil and gas or like operations, or both, are carried out. In its more specific aspects, the invention concerns a floating structure having buoyancy means to float the structure and in which the structure is anchored by a plurality of essentially parallel and vertical conduits commonly called "risers." More specifically, the invention concerns such a structure in which concentric casing strings, within riser pipes, form an important part of the anchoring system.
2. Setting
In recent years, it has become desirable to use a floating vessel from which to drill wells in marine locations. Many of these structures have been maintained on station by conventional spread catenary mooring lines, or by propulsion thruster units. One system of floating vessel receiving attention for drilling or production of wells in water is the Vertically Moored Platform, such as described in U.S. Pat. No. 3,648,638, issued Mar. 14, 1972, entitled "Vertically Moored Platform," Kenneth A. Blenkarn, inventor. A key feature of Vertically Moored Platforms is that the floating platform is connected to anchor means in the ocean floor only by elongated, parallel members which are preferred to be large diameter conduits, commonly called "riser pipes." These elongated members or riser pipes are held in tension by excess buoyancy of the platform.
3. Prior Art
This invention is an improvement over the anchoring system described in U.S. Pat. No. 3,648,638, supra. This patent is considered the closest prior art and, as stated above, our present invention is an improvement thereon. Other patents dealing with Vertically Moored Platforms include U.S. Pat. Nos. 3,559,410; 3,559,411; 3,572,272; 3,976,021; 3,978,804; 3,983,828; 3,993,273; 4,062,313; and 3,154,039. There are prior patents and art which teach to have concentric strings of casing extending from an underwater well to a platform above the water. In this latter regard, attention is directed to U.S. Pat. No. 3,971,576. U.S. Pat. No. 3,705,623 shows concentric pipes 33 and 17 connected to a buoyancy member 19; however, those concentric pipes form no part of the anchoring system. None of these patents or art to our knowledge teach to anchor a Vertically Moored Platform by means of concentric tensioned casing strings within an outer tensioned riser pipe. No prior art is known to do this.